1. Field of the Invention
The present invention relates to an occupant detection system and a method of controlling the occupant detection system.
2. Description of Related Art
JP-A-11-271463 discloses an occupant detection system having a mat-shaped electrostatic sensor and an electronic control unit (ECU). When a weak electric field is generated between a vehicle chassis and a main electrode disposed in an occupant seat, the electrostatic sensor outputs a variation of the electric field as current or voltage.
The electrostatic sensor has a pair of electrodes. When the occupant seat is vacant, air is filled between the pair of electrodes. When a child restraint system (CRS) is mounted on the occupant seat, the CRS is located between the pair of electrodes. When an occupant is seated on the occupant seat, the occupant is located between the pair of electrodes.
Air has a relative permittivity of about 1. The CRS has a relative permittivity of about 2-5. A body of the occupant has a relative permittivity of about 50. Thus, the relative permittivity is different among air, the CRS and the occupant body. Therefore, a capacitance between the pair of electrodes of the electrostatic sensor is changed, when a situation of the occupant seat is changed.
The variation of the electric field can be output by the change of the capacitance as current or voltage. The ECU performs a determination relative to the situation based on the output current/voltage value. That is, the ECU determines the situation of the occupant seat to be in a vacancy state, CRS-mounted state or occupant-seated state.
Further, an airbag electronic control unit determines an airbag inflation to be allowable or not based on the situation determination. Specifically, when the occupant seat is vacant, or when the CRS is mounted on the occupant seat, the airbag inflation is prohibited. When the occupant is seated on the occupant seat, the airbag inflation is allowable.
When the airbag inflation is allowable, an airbag ON-light is on to indicate the allowable state. When the airbag inflation is prohibited, an airbag OFF-light is on to indicate the prohibited state.
When the occupant seat is vacant, both of the ON-light and the OFF-light may be required to be off, because nothing exists on the occupant seat.
However, the capacitance of the vacancy state is close to that of the CRS-mounted state. Even when a one-year-old child, for example, is seated on the CRS, the capacitance is close between the CRS-mounted state and the vacancy state. Therefore, it is difficult to distinguish the situation of the occupant seat to be in the vacancy state or the CRS-mounted state.